A 15-year-old Indiana girl so malnourished that she weighed just 35 pounds when she was hospitalized Monday remains in critical condition with life-threatening injuries, police said.

The teen was near death Monday when she was rushed to an Indianapolis children's hospital from her home in Anderson. Investigators say she suffers from mental disabilities — and weighs the same as a typical 5-year-old.

"Last night we spoke with doctors, and they were telling us her recovery is going to be day-to-day," Anderson police Detective Joel Sandefur told NBC News on Thursday, adding that the girl is too weak to walk.

Police arrested the girl's grandfather and legal guardian — Steve Sells, 58, and his wife, Joetta Sue Sells, 54. Both are scheduled to appear via video at 11 a.m. Friday in Madison County court on child neglect charges. It wasn't immediately clear Tuesday morning if they had retained an attorney.

An ex-wife of Sells told NBC affiliate WTHR on Wednesday that she was stunned to learn of his arrest. "He loved kids. He loved my kids, my grandkids, any kid," said the woman, who didn't want to be identified. "He'd stop and talk to them, talk about how sweet they was, how cute they was. I mean, he loved kids."

But police allege Sells and his new wife hid horrific conditions inside their home. After other family members took the girl to the hospital, investigators discovered she had been living in a locked room with nothing more than a filthy mattress, a bucket and a bowl of oatmeal. Police also found blood stains.

Steve Sells, who weighs about 225 pounds, claimed that he locked in his granddaughter to protect himself because she was strong and threatened him with a knife. Sells also said a chromosome disorder prevented her from putting on weight. The family had been receiving $720 a month in government assistance for her care, Sandefur said.

Sells told police his daughter — the teen's biological mother — abandoned the girl. Sandefur said Thursday they were still trying to locate that woman.

Neighbors, meanwhile, said they were shocked to learn of the sickening conditions, and said the family had other children, including a 4-year-old. Steve Sells “made it seem like he was a loving, caring father that had took care of his kids and everything like that," neighbor Thomas McAllister told WTHR. "To find this out, it's just mind-boggling. It's repulsive and repugnant. It's just ... disgusting."